Method for amplitude modulation of a radio frequency signal, and device therefor

ABSTRACT

The invention provides a method and a device for amplitude modulation of a radio frequency signal using a variable-gain radio frequency power amplifier, for automatic control of the amplifier output signal by generating an error signal between an amplitude modulation signal and a signal representing the power of the amplifier output signal, from which a gain control signal of the amplifier is generated, to monitor potential saturation of the amplifier by comparing the relative phases of the error signal and of the amplitude modulation signal, and, in case of saturation of the amplifier, to generate a correction signal for lowering the operating point of the amplifier. The invention is applicable to a transmitter of a mobile station or a fixed station of a radio communication system.

TECHNICAL FIELD

The present invention relates to a method of amplifying and modulatingthe amplitude of a radiofrequency signal, to a device for implementingthe method and to a generator comprising such a device for generating aradiofrequency signal comprising a phase or frequency modulationcomponent and an amplitude modulation component, a radiofrequencytransmitter incorporating such a generator, and also a mobile stationand a fixed station of a radiocommunication system comprising such atransmitter.

The invention relates to the field of the techniques for amplifyingradiofrequency signals comprising, on the one hand, a phase or frequencymodulation component and, on the other hand, an amplitude modulationcomponent, which are suitable for radio transmission via an antenna or acable.

The invention finds applications in radiofrequency transmitters,especially mobile stations or fixed stations of a radiocommunicationsystem, for example a private mobile radiocommunication system (or PMR).

BACKGROUND OF THE INVENTION

In radiocommunication systems of this type, digital data encoding anaudio signal or, more generally, information of any type, aretransmitted by means of an amplitude modulation which takes place inaddition to a phase or frequency modulation of the transmittedradiofrequency signal. Thus, the transmitted radiofrequency signal hasboth a phase or frequency modulation component and an amplitudemodulation component. Adding an amplitude modulation component makes itpossible in general to improve the bit rate characteristics for a givenchannel width.

The output stage of the transmitter comprises a radiofrequency poweramplifier which, so as to obtain a high power efficiency (this beingparticularly required in the case of use of a transmitter in portableradiocommunication equipment), must operate within an operating regionclose to saturation.

Now, as is known, a power amplifier in such an operating region exhibitsamplification nonlinearities comprising amplitude nonlinearities andphase non-linearities. In the literature, these nonlinearities are oftendenoted by amplitude/amplitude conversions (or AM/AM conversions) oramplitude/phase conversions (or AM/PM conversions), respectively. Thesenon-linearities cause distortion of the transmitted signal, whichdistortion degrades the performance of the transmitter in terms oftransmission quality, this loss of quality generally resulting inundesirable broadening of the spectrum.

Various techniques have been proposed for eliminating the effects of theamplification nonlinearities of radiofrequency power amplifiers. Thesetechniques are called radiofrequency power amplifier linearizationtechniques. In particular, mention may be made of the CLLT (CartesionLoop Linear Transmitter) technique, the ABP (Adaptive BasebandPredistortion) technique and the EER (Envelope Elimination andRestoration) technique, etc.

The EER technique is very old, since it has been applied since the 50sfor amplification of single-sideband (SSB) radiofrequency signals.

The principle of the EER technique is illustrated in FIG. 1, which is asimplified diagram of a radiofrequency signal generator that relies onthis technique. The modulation of the radiofrequency signal G output bythe generator is decomposed into a phase or frequency modulationcomponent on the one hand, and an amplitude modulation component on theother. These two components are generated in baseband.

In the example shown, a phase modulation component B is delivered, asphase or frequency modulation signal, to the input of phase or frequencymodulation means MOD, for example comprising a phase modulator, whichtranspose it into the radiofrequency range. In a variant of the EERtechnique, known as the OPLEER (Open Phase Loop EER) technique, themeans MOD comprise a phase-locked loop. Such phase modulation means haveextremely low broadband noise characteristics because of the highspectral purity that the phase-locked loop can achieve.

The signal E output by the modulation means MOD is a phase-modulatedsignal of approximately constant amplitude. This signal is thenamplified by a radiofrequency power amplifier PA.

An amplitude modulation component C is delivered, as amplitudemodulation signal, via circuits (not shown in FIG. 1), to a gain controlinput of the amplifier PA in order to control the gain of thisamplifier. This mechanism allows the amplitude modulation component tobe reintroduced into the amplified radiofrequency signal withoutinjecting additional noise. The amplifier PA may be a component having again control input or a group of components having a gain control input.

Thus, the amplitude modulation component is super-imposed on the phasemodulation component in order to obtain the desired radiofrequencysignal G as the output of the amplifier PA, these two components usingdifferent paths to reach the output of the amplifier PA.

A radiofrequency transmitter relying on the OPLEER technique isdescribed, for example, in French patent application FR 2 716 589. Thistransmitter includes AM/AM conversion correction means and AM/PMconversion correction means for the radiofrequency power amplifier PA,in the form of an output signal amplitude control loop and an outputsignal phase control loop, respectively, which are imbricated.

The diagram of FIG. 2 shows AM/AM conversion correction means for theradiofrequency power amplifier PA, in the case of a generator of thetype shown in FIG. 1, which are described in the aforementioned documentFR 2 716 589.

These means comprise, for example, an analog control loop for feedbackcontrol of the output signal G to the amplitude modulation signal C. Theanalog control loop includes a comparator amplifier COMP, a first inputof which receives the amplitude modulation signal C, a second input ofwhich receives a signal L and the output of which delivers an amplitudecontrol signal F. The latter signal is applied to a gain control inputof the amplifier PA. The output of the amplifier COMP is fed back to itssecond input via an impedance, such as a capacitor C so as to preventspurious oscillations of the signal F. The signal L is an analog signalrepresentative of the power of the output signal G.

The analog control loop further includes coupling means, such as aradiofrequency coupler 4, for extracting part of the energy of theoutput signal G and delivering a signal H that is the image of theoutput signal G.

Finally, it includes a detector DET, the input of which receives thesignal H and the output of which delivers the aforementioned signal L.The detector DET allows the amplitude modulation component of the outputsignal G to be extracted from the signal H by applying rectification andlow-pass filtering to the signal H so that the voltage amplitude of thesignal L, conventionally expressed in decibel-volts (dBv), is a functionof the instantaneous power of the signal H, conventionally expressed indecibels (dBm). The signal L is therefore representative of theamplitude modulation component actually present in the output signal G.

The signal L and the amplitude modulation signal C are very close toeach other and differ only by the effect of the AM/AM conversions in theamplifier PA. The signal L is compared to the amplitude modulationsignal C by the comparator amplifier COMP, which produces the amplitudecontrol signal F on the basis of their difference.

These AM/AM conversion correction means have the drawback of notcontrolling the operating point of the amplifier PA. Now, should therebe a variation in the supply voltage or in the temperature, for example,the operating point of the amplifier PA may be shifted toward thesaturation region of the amplifier PA. Saturation of the amplifier PAgenerates a distortion of the output signal, without this distortionbeing detected, or a fortiori corrected.

The graph in FIG. 3 a shows the variation in the amplitude of the outputsignal G in a normal operating case, that is to say when the operatingpoint of the amplifier PA is such that, despite the required amplitudemodulation, the entire amplitude variation remains below the saturationpoint. The graph shown in FIG. 3 b gives the corresponding spectrum ofthe output signal G. As a result of a temperature variation or of avariation in the supply voltage, the operating point may be shiftedtoward the saturation region of the power amplifier. In certain cases,the analog control loop can no longer provide a sufficient excursion ofthe amplitude control signal F in order to obtain an output signal Ghaving the required amplitude variation. The amplitude variation shownin the graph of FIG. 3 c is then obtained. The corresponding spectrum isshown in the graph of FIG. 3 d. As may be seen by comparing theseFigures to FIGS. 3 a and 3 b, the distortion by the clipping of theamplitude then results in spectral broadening of the output signal G.

SUMMARY OF THE INVENTION

The invention thus proposes a method of modulating the amplitude of aradiofrequency signal using a variable-gain radiofrequency poweramplifier, which makes it possible to detect saturation of the amplifierPA and to lower the operating point of the amplifier so as to reduce themean power output by the amplifier when the latter saturates.

More particularly, the invention proposes a method of modulating theamplitude of a radiofrequency signal using a radiofrequency poweramplifier, comprising the steps consisting:

-   -   a) in applying feedback control to the power of the output        signal of the amplifier by generating an error signal between an        amplitude modulation signal and a signal representative of the        power of the output signal of the amplifier, on the basis of        which a signal for controlling the gain of the amplifier is        generated;    -   b) in monitoring potential saturation of the amplifier by        comparing the relative phases of the error signal and the        amplitude modulation signal; and    -   c) should the amplifier saturate, in generating a correction        signal for lowering the operating point of the amplifier.

The invention further proposes a device for implementing the method,comprising:

-   -   a variable-gain radiofrequency power amplifier;    -   means for applying feedback control to the power of the output        signal of the amplifier, which means generate a signal for        controlling the gain of the amplifier on the basis of an error        signal between an amplitude modulation signal and a signal        representative of the power of the output signal of the        amplifier;    -   means for comparing the relative phases of the error signal and        the amplitude modulation signal so as to deduce therefrom        potential saturation of the amplifier; and    -   means for, should the amplifier saturate, generating a        correction signal for lowering the operating point of the        amplifier.

The invention applies in particular to the recovery of the amplitudemodulation component by controlling the gain of a radiofrequency poweramplifier in a generator relying on the EER technique or on the OPLEERtechnique, without however its scope being limited to this application.

The invention thus proposes a generator for generating a radiofrequencysignal having a phase modulation component and an amplitude modulationcomponent, which comprises:

-   -   means for generating a phase or frequency modulation signal and        an amplitude modulation signal;    -   phase or frequency modulation means comprising an input that        receives the phase or frequency modulation signal and an output        that delivers a phase-modulated or frequency-modulated        radiofrequency signal of approximately constant amplitude; and    -   a device as defined above for modulating the amplitude of the        radiofrequency signal delivered by the phase or frequency        modulation means on the basis of the amplitude modulation        signal.

The invention further proposes a radiofrequency transmitter, for examplea mobile station or fixed station transmitter of a PMR system,comprising such a generator.

Finally, it proposes a mobile station and a fixed station of aradiocommunication system, for example a PMR system, which comprise sucha transmitter, and also a radiocommunication system, for example a PMRsystem, comprising at least one such mobile station and at least onesuch fixed station.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, already discussed is a diagram illustrating the principle of theEER or OPLEER technique;

FIG. 2, also already discussed is a diagram showing an analog controlloop for the amplitude of the output signal of the radiofrequency poweramplifier;

FIGS. 3 a and 3 b on the one hand and FIGS. 3 c and 3 d on the other,again already discussed, are graphs showing the variation in amplitudeand the spectrum of the output signal in the absence of saturation andin the presence of saturation of the amplifier, respectively;

FIGS. 4 a to 4 c are graphs showing a constellation, the spectrum andthe amplitude variation, respectively, of a radiofrequency signal havinga phase or frequency modulation component and an amplitude modulationcomponent;

FIG. 5 is the graph of the output power as a function of a gain controlsignal for a radiofrequency power amplifier used in the device accordingto the invention;

FIG. 6 is the diagram of a radiofrequency signal generator comprising adevice according to the invention;

FIG. 7 is a graph illustrating the effects of a shift in the operatingpoint of the radiofrequency power amplifier of a device according to theinvention;

FIGS. 8 a to 8 f are graphs illustrating the comparison of the relativephases of the error signals and of the phase modulation signal accordingto the invention;

FIG. 9 is the diagram of a first illustrative example of a synchronousdetector according to the invention;

FIG. 10 is the diagram of a second illustrative example of a synchronousdetector according to the invention;

FIG. 11 is the graph of the variation of a weighting parameter usedaccording to the invention;

FIG. 12 is a flowchart for the steps of the method according to theinvention;

FIG. 13 is the diagram of a radiofrequency transmitter according to theinvention; and

FIG. 14 is the diagram of a radio-communication system according to theinvention.

DETAILED DESCRIPTION

The device according to the invention is shown schematically in FIG. 6,in its application to a generator for generating a radiofrequency signalcomprising a phase or frequency modulation component and an amplitudemodulation component and being suitable for radio transmission via anantenna or a cable, which is based on the OPLEER technique.

The device according to the invention is advantageously, but notnecessarily, incorporated into such a generator. The OPLEER technique,which was presented in the introduction with regard to the diagram ofFIG. 2, is particularly suitable for the transposition toradiofrequencies and the power amplification of baseband signals havinga low depth of amplitude modulation, that is to say a low amplitudevariation.

An example of such a baseband signal is shown in the graphs of FIGS. 4a, 4 b and 4 c. These Figures show the constellation, the spectrum andthe amplitude variation (centred about a zero mean value) of the signal,respectively. This signal is, for example, obtained by phase andquadrature filtering (I and Q filtering) of a pulse-width modulatedsignal (code pulse modulation or CPM). It constitutes a good compromisebetween spectral occupancy and sensitivity relative to the noise. Thetotal amplitude variation does not exceed ±1 dB, with a ratio of peakpower to rms power of less than ±1 dB.

The generator includes means for generating a phase or frequencymodulation signal B and an amplitude modulation signal C.

In the example described below, a phase modulation component isenvisioned that is obtained by frequency conversion on the basis of theI and Q components of the corresponding baseband signal. This componentis thus defined by the phase or frequency modulation signal B, which isan analog signal. However, it should be noted that the phase modulationcomponent may also be obtained by direct digital generation, in whichcase the signal B would be a digital signal. Likewise, the example of anamplitude modulation component, which is obtained by rectifying thecorresponding baseband signal, may be envisioned. This component is thendefined by the amplitude modulation signal C, which is an analog signal.However, it should be noted that this component may also be obtained bydirect digital generation, in which case the signal C would be a digitalsignal. In practice, the signals B and C are delivered to the generatorby upstream encoding means.

The amplitude modulation signal C contains a DC component V_(C)=, whichallows the mean power level output by the amplifier PA to be defined,and an AC component V_(C)˜, which defines the actual amplitudemodulation.

For producing the phase or frequency modulation means MOD, the generatorcomprises a phase-locked looped as described in document FR 2 716 589cited in the introduction.

To produce the radiofrequency power amplifier PA, the deviceadvantageously comprises a field-effect transistor (FET) havingcharacteristics such as those shown in the graph of FIG. 5. An amplifierof this type has the advantage, firstly, of having a curve of the outputpower Po that is linear over a wide gain control voltage range and,secondly, of having a total efficiency E_(t) that reaches its maximumvalue for a control voltage corresponding to an operating pointsufficiently away from the saturation point. In this way, a signal ofthe type illustrated in FIGS. 4 a, 4 b and 4 c may be amplified withoutthe overall efficiency of the amplification being reduced relative tothe amplification of a constant-amplitude signal at the saturation pointor beyond.

The device further includes a comparator amplifier COMP, operating as anerror amplifier. This is, for example, an operational amplifier. Theamplifier COMP produces an error signal U, namely the error between theamplitude modulation signal C and a signal L that is representative ofthe power of the output signal G of the amplifier PA. The signal L isproduced by a detector DET on the basis of a signal H extracted from theoutput of the amplifier PA by a coupler 4. The inverting input of theamplifier COMP, which receives the signal L (in this case via a summingamplifier 7 to which reference will be made later), is connected to itsoutput via two capacitors C2 and C3 in series. These capacitors blockthe spurious oscillations of the output of the amplifier COMP.

The signal U may be applied to a gain control input of the amplifier PA.The aforementioned elements of the device according to the inventiontherefore form means for applying feedback control to the power of theoutput signal G of the amplifier PA, which means are known per se andare similar to the corresponding elements of the generator shown in FIG.2.

However, in a variant shown, means for applying feedback control to thepower of the output signal G of the device according to the inventionfurther include a summing amplifier 5, operating as an analog adder. Theamplifier 5 produces a signal F for controlling the gain of theamplifier PA, which signal is supplied to the gain control input of theamplifier PA. The signal F is produced by the amplifier 5 by adding theerror signal U, on the one hand, to the amplitude modulation signal C ora signal P, on the other hand, which signal P is obtained from saidsignal C by applying to it a predistortion by means of a predistortionmodule PD. The advantage of the predistortion module PD is as follows.When an amplitude modulation signal is applied to the input of the gaincontrol of the amplifier PA, this creates a distortion by nonlinearity.This distortion may be at least partly compensated for by applying apredistortion to the amplitude modulation signal, which compensates forthese nonlinearities.

The signal P output by the predistortion module PD, that is to say theamplitude modulation signal C with predistortion, is a signal that is inphase with the signal C. It corresponds to the control voltage that mustbe applied to the gain control input of the amplifier PA under nominalconditions so that the power of the output signal G corresponds to thedesired value.

The graph of FIG. 7 shows the characteristic of the amplifier PA, thatis the curve of the output power Po as a function of the control voltageV_(P) applied to the gain control input. This graph illustrates the roleof the means for applying feedback control to the amplitude of theoutput signal G.

The aforementioned nominal conditions correspond to an operating pointFP0. Under these nominal conditions, to obtain an output signal G with agiven amplitude variation, shown in FIG. 7 by curve 20, it is necessaryto apply a control signal F, having a given amplitude variation shown inFIG. 7 by curve 10, to the gain control input of the amplifier PA.

If, however, for example because of a variation in the supply voltage orin the temperature, the operating point of the amplifier PA moves up thecharacteristic, the slope of the function Po=f(V_(P)) decreases. Thus,for example, at the operating point labeled PF2 on the graph in FIG. 7,a control signal F having the amplitude variation shown by curve 12,which is greater than that shown by curve 10, has to be applied to thegain control input of the amplifier PA. Conversely, if the operatingpoint of the amplifier PA moves down the characteristic, the slope ofthe function Po=f(V_(P)) increases. As a result, for example at theoperating point labeled PF1 on the graph of FIG. 7, a control signal Fhaving the amplitude variation shown by curve 11, which is smaller thanthat shown by curve 10, has to be applied to the gain control input ofthe amplifier PA. The means for applying feedback control to theamplitude of the output signal G normally make it possible to generatethe amplitude control signal F that has to be applied to the gaincontrol input of the amplifier PA in order to obtain the desiredamplitude variation of the output signal G.

According to one finding, which is at the basis of the invention, it ispossible, by observing the relative phases of the error signal U on theone hand and of the amplitude modulation signal with predistortion(signal P) or without predistortion (signal C) on the other, to make thefollowing comments. When these two signals are in phase opposition, theamplitude variation of the amplitude modulation signal is greater thanthe value normally required at the operating point PF0. This is becausethe error signal U then has the effect of reducing the amplitude controlsignal F, ensuring that the power of the output signal G complies withthe desired value. The effective operating point is in this case in alower position than that of the operating point PF0 under the nominalconditions. Conversely, if the error signal U on the one hand, and theamplitude modulation signal with predistortion (signal P) or withoutpredistortion (signal C) on the other, are in phase, the amplitudevariation of the amplitude modulation signal is greater than the valuenormally required at the operating point PF0. This is because the errorsignal U then tends to increase the amplitude control signal F. Theeffective operating point is in this case in a higher position than thatof the operating point PF0 under the nominal conditions.

If the effective operating point is located too high up on thecharacteristic of the amplifier PA, the error signal U may potentiallybe unable to achieve an excursion sufficient to provide the amplitudevariation of the control signal F to be applied to the gain controlinput of the power amplifier. The effective operating point is then inthe saturation region. In other words, the amplifier PA is saturated.

As a consequence of the above findings, the invention suggests that therelative phases of the error signal U and the phase modulation signal C,or the phase modulation signal with predistortion P, be compared so asto deduce therefrom any potential saturation of the amplifier PA.

For this purpose, the device according to the invention further includesmeans for comparing the relative phases of the error signal U and theamplitude modulation signal C. These means comprise means for thesynchronous detection of the AC component V_(U)˜ of the error signal Urelative to the AC component V_(C)˜ of the amplitude modulation signalC, which means produce a signal J. They further include means forcomparing the signal J to a threshold, which comparison means, shouldamplifier PA be saturated by the threshold being exceeded, generate acorrection signal W for lowering the operating point of the amplifierPA.

The synchronous detection means comprise a synchronous detector SDhaving at least two inputs and one output. A first of these inputsreceives the AC component V_(U)˜ of the error signal U that is extractedby the capacitor C2. A second of these inputs receives the AC componentV_(C)˜ of the amplitude modulation signal C via a capacitor C1. Theoutput delivers the aforementioned signal J.

The graphs of FIGS. 8 a, 8 b and 8 c provide a representation of the ACcomponent V_(C)˜ of the amplitude modulation signal C, the AC componentV_(U)˜ of the error signal U and the signal J output by the synchronousdetector SD, respectively, in the case in which the two aforementionedAC components are in phase opposition. The graphs of FIGS. 8 d, 8 e and8 f provide an equivalent representation in the opposite case, in whichthe aforementioned two AC components are in phase.

As may be seen in these graphs, the signal J output by the synchronousdetector SD corresponds to the error signal U during the positivehalf-cycles of the amplitude modulation signal C and, conversely, to theerror signal U during the negative half-cycles of the amplitudemodulation signal C. As will have been understood, it is determined thatthe amplifier PA has saturated when the signal J is positive and above acertain threshold.

The means for comparing the signal J to a threshold, which are used forthis determination, comprise, for example, a comparator 6 or a Schmitttrigger or the like. The output signal of the comparator 6 is filteredby a smoothing capacitor C4 or an integrator circuit or the like. Thecorrection signal W is the signal at the terminals of this smoothingcapacitor C4.

The threshold of the comparator 6 depends on the nominal operating pointPF0 of the amplifier PA, which is determined by the DC component V_(C)=of the amplitude modulation signal C.

The device further includes means, such as the summing amplifier 7mentioned earlier, which operates as an anlog adder in order to add thecorrection signal W to the signal L representative of the power of theoutput signal G of the amplifier PA. Thus, the correction signal W,which is a DC signal, has the effect of simulating an increase in the DCcomponent of the output signal G, in such a way that the means forapplying feedback control to the amplitude of the output signal G act bylowering the operating point of this amplifier.

The diagram of FIG. 9 shows an illustrative example of the synchronousdetector SD.

In this example, the detector includes a two-position switch 90. In thefirst position 91, the switch 90 delivers the AC component V_(U)˜ of theerror signal U, received on a first input 95 of the detector, to theoutput 98 of the detector. In the second position 92, the switch 90delivers this AC component V_(U)˜ to the output 98 after said ACcomponent V_(U)˜ has been inverted by means of an inverter amplifier 93.The inverter amplifier 93 is, for example, a circuit that includes anoperational amplifier, the gain of which is equal to −1.

The switching of the switch 90 from one of the positions 91 and 92 tothe other is controlled by an activation signal delivered by acomparator amplifier 94 that receives, as input, the DC component V_(C)=and the AC component V_(C)˜ of the amplitude modulation signal C, thesecomponents being received on a second input 96 and a third input 97 ofthe detector, respectively.

It should be noted that the DC component V_(C)= of the amplitudemodulation signal C can be easily obtained, for example by subtractingthe AC component V_(C)˜ from the amplitude modulation signal C by meansthat are not shown but are within the competence of a person skilled inthe art.

The diagram of FIG. 10 shows another illustrative example of thesynchronous detector SD.

In this example, the synchronous detector SD includes a symmetricalmultiplier 100 with two Gilbert cells. Such a multiplier has four inputs105, 106, 107 and 108 and one output 109. The output 109 of thesymmetrical multiplier delivers the signal J. The symmetrical multiplier100 is preceded by a matching stage (not shown) similar to that used inquadrature detectors serving to demodulate the frequency modulation.

The AC component V_(U)˜ of the error signal U, received on a first input103 of the detector SD, is applied to the input 105 and, via aninverting amplifier 101, to the input 106 of the symmetrical multiplier100. In addition, the AC component V_(C)˜ of the amplitude modulationsignal C, received on a second input 104 of the detector SD is appliedto the input 107 and, via another inverting amplifier 102, to the input108 of the symmetrical multiplier 100.

The inverting amplifiers 101 and 102 are, for example, produced byrespective circuits each comprising an operational amplifier, the gainof which is equal to −1.

In an advantageous embodiment, the means for applying feedback controlto the power of the output signal G of the amplifier PA are designed toproduce the signal F for controlling the gain of the amplifier PA byadding the error signal U, after this has been weighted by a weightingparameter k, for example a multiplicative parameter, to the amplitudemodulation signal C (or to the signal P corresponding to the amplitudemodulation signal C with predistortion). This weighting is obtained byadapting the adding amplifier 5 in such a way that it produces thefunction C+k.U (or P+k.U). The weighting parameter k depends on thenominal operating point of the amplifier PA, as determined by the DCcomponent V_(C)= of the amplitude modulation signal C.

The typical variation in the value of the weighting parameter k as afunction of the DC component V_(C)= of the amplitude modulation signal Cis given by the graph of FIG. 11.

For the purposes of the present invention and in what follows, theexpression “low values of the DC component V_(C)= of the amplitudemodulation signal C” is understood to mean values that are lower than asecond defined value V1 below which the detector DET is not linear. Inaddition, the expression “high value of the DC component V_(C)= of theamplitude modulation signal C” is understood to mean values that aregreater than a second defined value V2, above which the gain of theamplifier PA starts to decrease.

For V_(C)= values between the values V1 and V2, the parameter k ispreferably equal to unity (k=1) in such a way that the signal U and thesignal C (or the signal P) are added without weighting.

For high values of V_(C)=, that is to say for values of the mean powerof the output signal G that are close to saturation of the poweramplifier PA, the value of the parameter k increases so as to compensatefor the loss of gain of the amplifier PA (which is a manifestation ofthe reduction in the slope of the characteristic, shown in FIG. 7, ofthe amplifier PA). In this way, the loop gain is kept constant, makingit easier to produce the low-pass loop filter (not shown) whose cutofffrequency would otherwise vary with the mean power of the output signalG.

For low values of V_(C)=, the value of k decreases and becomes zero.Thus, the feedback via the loop comprising the coupler 4 and thedetector DET is not used in a region in which the detector might havedeviations from linearity and in which the amplifier PA does not havesuch. Thus, it is possible to use only a detector whose detectionlinearity region is small, the predistortion itself providing thenecessary corrections in the region in which the power amplifier isvirtually linear.

The steps of the method according to the invention are summarized andillustrated by the flowchart of FIG. 12.

In step 73, feedback control is applied to the power of the outputsignal G of the amplifier PA by generating, in substep 71 of step 73,the error signal U between the amplitude modulation signal C and thesignal L representative of the power of the output signal G of theamplifier PA. In substep 72 of step 73, a signal F for controlling thegain of the amplifier PA is generated on the basis of the error signalU. In a first example, the control signal F is generated by adding theerror signal U to the amplitude modulation signal C. In a variant ofthis example, the error signal U is added to the amplitude modulationsignal C after predistortion of the latter, that is to say that it is infact added to the signal P delivered by the predistortion module PD.

In step 76, potential saturation of the amplifier PA is monitored bycomparing the relative phases of the error signal U and the amplitudemodulation signal C. Should the amplifier PA saturate, a correctionsignal W is generated in step 77, allowing the operating point of theamplifier PA to be lowered.

In one example, the saturation of the amplifier PA is monitored bysubstep 74 of step 76, which consists in carrying out synchronousdetection of the AC component V_(U)˜ of the error signal U relative tothe AC component V_(C)˜ of the amplitude modulation signal C, and bysub-step 73 of step 76, which consists in comparing the signal producedby this detection (this signal is shown in FIGS. 8 c and 8 f) to athreshold so as to generate, in step 77, the correction signal W whenthis threshold is exceeded.

Typically, the threshold depends on the DC component V_(C)= of theamplitude modulation signal C.

Advantageously, the correction signal W is added to the signal Lrepresentative of the power of the output signal G of the amplifier PAso as to cause the means for applying feedback control to the amplitudeof the output signal G to act in order to lower the operating point ofthe amplifier PA and thus move away from the saturation region of thelatter.

Before being added to the amplitude modulation signal C, the errorsignal U may be weighted by a weighting parameter k that depends on theDC component V_(C)= of the amplitude modulation signal C.

FIG. 13 is the diagram of a radiofrequency transmitter according toembodiment of the present invention.

The transmitter 60 includes a data input 100 for receiving a digitalmessage A containing data to be transmitted. When the transmitter isused in a mobile station or a fixed station of a radiocommunicationsystem, the input 100 may be connected to the output of a speech encoderor of a channel encoder.

The transmitter also includes composite encoding means, such as anencoder 200 for generating, on the basis of the digital message A, afirst series of digital values constituting the phase modulation signalB and a second series of digital values constituting the amplitudemodulation signal C. In this example, the signal B and the signal C aretherefore digital signals.

The transmitter further includes, downstream of the encoder 200, agenerator 300 for generating a radiofrequency signal having a phasemodulation component and an amplitude modulation component, as describedabove with regard to FIG. 6.

Finally, the transmitter includes a radiofrequency antenna 400 connectedto the output of the generator 300. This antenna transmits thephase-modulated and amplitude-modulated radiofrequency signal G into thetransmission channel. As a variant, the antenna 400 may be replaced witha cable.

FIG. 14 shows schematically a radiocommunication system according to oneembodiment of the present invention. The system 70 includes a networksubsystem, shown symbolically by a cloud 73. It also includes a radiosubsystem, comprising mobile stations 71 and/or fixed stations 72. Themobile stations 71 are, for example, portable terminals. The fixedstations 72 are, for example, base stations that provide the radiointerface with the mobile stations that fall within their radio coveragearea. As a variant, they may be fixed terminals.

At least one fixed station 71 and/or at least one mobile station 72 ofthe system 70 are equipped with a radiofrequency transmitter 60according to the diagram of FIG. 12.

1. A method of modulating the amplitude of a radio frequency signalusing a radiofrequency power amplifier, comprising: a) applying feedbackcontrol to the power of the output signal of the amplifier by generatingan error signal between an amplitude modulation signal and a signalrepresentative of the power of the output signal of the amplifier, onthe basis of which a signal for controlling the gain of the amplifier isgenerated; b) monitoring potential saturation of the amplifier bycomparing the relative phases of the error signal and the amplitudemodulation signal; and c) should the amplifier saturate, then generatinga correction signal for lowering the operating point of the amplifier.2. The method of claim 1, wherein the saturation of the amplifier ismonitored by synchronous detection of the AC component of the errorsignal relative to the AC component of the amplitude modulation signaland comparison of the signal thus detected to a threshold in order togenerate the correction signal when the threshold is exceeded.
 3. Themethod of claim 2, wherein the threshold depends on the DC component ofthe amplitude modulation signal.
 4. The method of claim 1, wherein thecorrection signal is added to the signal representative of the power ofthe output signal of the amplifier.
 5. The method of claim 1, whereinthe signal for controlling the gain of the power amplifier is generatedby adding the error signal to the amplitude modulation signal.
 6. Themethod of claim 5, wherein the error signal is weighted by a weightingparameter before being added to the amplitude modulation signal, saidweighting parameter depending on the DC component of the amplitudemodulation signal.
 7. The method of claim 6, wherein the weightingparameter is a multiplicative parameter that tends toward zero for lowvalues of the DC component of the amplitude modulation signal.
 8. Themethod of claim 6, wherein the weighting parameter is a multiplicativeparameter that increases for high values of the DC component of theamplitude modulation signal so as to compensate for the loss of gain ofthe amplifier.
 9. The method of claim 1 wherein the error signal isadded to the amplitude modulation signal after predistortion of thelatter.
 10. A device for modulating the amplitude of a radiofrequencysignal, comprising: a variable-gain radiofrequency power amplifier;means for applying feedback control to the power of the output signal ofthe amplifier, which means generate a signal for controlling the gain ofthe amplifier on the basis of an error signal between an amplitudemodulation signal and a signal representative of the power of the outputsignal of the amplifier; means for comparing the relative phases of theerror signal and the amplitude modulation signal so as to deducetherefrom potential saturation of the amplifier; and means for, shouldthe amplifier saturate, generating a correction signal for lowering theoperating point of the amplifier.
 11. The device of claim 10, whereinsaid means for comparing the relative phases of the error signal and theamplitude modulation signal comprise means for the synchronous detectionof the AC component of the error signal relative to the AC component ofthe amplitude modulation signal, and means for comparing the signalproduced by these detection means to a threshold, which generate thecorrection signal when the threshold is exceeded.
 12. The device ofclaim 11, wherein the threshold depends on the DC component of theamplitude modulation signal.
 13. The device of claim 10, furthercomprising means for adding the correction signal to the signalrepresentative of the power of the output signal of the amplifier. 14.The device of claim 10, further comprising means for generating thesignal for controlling the gain of the amplifier by adding the errorsignal to the amplitude modulation signal.
 15. The device of claim 14,further comprising means for predistorting the amplitude modulationsignal which is added to the error signal.
 16. The device of claim 10,wherein the means for applying feedback control to the power of theoutput signal of the amplifier are designed to produce the signal forcontrolling the gain of the amplifier by adding the error signal to theamplitude modulation signal after the error signal has been weighted bya multiplicative parameter, said weighting parameter depending on the DCcomponent of the amplitude modulation signal.
 17. The device of claim16, wherein the weighting parameter is a multiplicative parameter thattends toward zero for low values of the DC component of the amplitudemodulation signal.
 18. The device of claim 16, wherein the weightingparameter is a multiplicative parameter that increases for high valuesof the DC component of the amplitude modulation signal so as tocompensate for the loss of gain of the amplifier.
 19. A generator forgenerating a radiofrequency signal having a phase modulation componentand an amplitude modulation component, which comprises: means forgenerating a phase or frequency modulation signal and an amplitudemodulation signal; phase or frequency modulation means comprising aninput that receives the phase or frequency modulation signal and anoutput that delivers a phase-modulated or frequency-modulatedradiofrequency signal of approximately constant amplitude; and a deviceas claimed in any one of claims 10 to 18 for modulating the amplitude ofthe radiofrequency signal delivered by the phase or frequency modulationmeans on the basis of the amplitude modulation signal.
 20. Aradiofrequency transmitter, comprising a generator as claimed in claim19.
 21. A mobile station of a radiocommunication system, comprising atransmitter as claimed in claim
 20. 22. A radiocommunication system,comprising a mobile station as claimed in claim
 21. 23. A fixed stationof a radiocommunication system, comprising a transmitter as claimed inclaim
 20. 24. A radiocommunication system, comprising a fixed station asclaimed in claim 23.